Exclude current month

Hi Oros ,
You just nned to apply filter in your DAX Measure 

Sales_Excluding_Current_Month = 
VAR CurrentMonth = MONTH(TODAY())
VAR CurrentYear = YEAR(TODAY())
RETURN
CALCULATE(
    SUM(Sales[SalesAmount]),
    FILTER(
        Sales,
        YEAR(Sales[Date]) < CurrentYear ||
        (YEAR(Sales[Date]) = CurrentYear && MONTH(Sales[Date]) < CurrentMonth)
    )
)

If there is any post helps, then please consider Accept it as the solution  to help the other members find it more quickly.
If I misunderstand your needs or you still have problems on it, please feel free to let us know. Thanks a lot!

Oros 
you should use a date table/Calender table and use the measure below.

Total Sales =
VAR CurrentMonth = MAX('Date'[Date])
VAR LastMonth = CALCULATE(MAX('Date'[Date]), ALL('Date'))
RETURN
IF(
CurrentMonth = LastMonth,
BLANK(), -- Exclude current month if it's the last available month
CALCULATE(
SUM('Sales'[SalesAmount]),
FILTER(
ALL('Date'),
'Date'[Date] <= LastMonth -- Include months up to the last completed month
)
)
)

Did I answer your question? Mark my post as a solution! Appreciate your Kudos !!